The general method of operation of such rotary valves is well known in the art of power steering design and so will not be described in any greater detail in this specification. A description of this operation is contained in U.S. Pat. No. 3,022,772 (Zeigler), commonly held as being the "original" patent disclosing the rotary valve concept.
Such rotary valves are nowadays regularly incorporated in firewall-mounted rack and pinion steering gears and, in this situation, any noises such as hiss emanating from the valve are very apparent to the driver. Hiss results from cavitation of the hydraulic oil as it flows in the orifices defined by the input-shaft metering edge contours and the adjacent edges of the sleeve slots, particularly during times of high pressure operation of the valve such as during vehicle parking manoeuvres. It is well known in the art of power steering valves that an orifice is less prone to cavitation if the metering edge contour has a high aspect ratio of width to depth, thereby constraining the oil to flow as a thin sheet of constant depth all along any one metering edge contour. Similarly it is important that the flow of oil divides equally amongst the aforementioned network of orifices, so further effectively increasing the above aspect ratio. This requires highly accurate angular spacing of the input-shaft metering edge contours as well as the precision of manufacture of each metering edge contour to ensure uniformity of depth along their length. Precision is most important in that portion of the metering edge contour controlling high pressure operation of the rotary valve associated with parking manoeuvres, where the pressure generated is typically 8 MPa and the metering edge contour depth only about 0.012 mm. This portion lies immediately adjacent to the outside diameter of the input-shaft, and is associated with the maximum normal operating angle of the valve. However precision is also required in order to avoid hiss further down the metering edge contour where the pressure generated is typically 2 MPa and the contour depth about 0.024 mm. The remainder of the metering edge contour towards the centred position of the rotary valve is important in determining the valve pressure characteristic, but not valve noise.
It is also well known that cavitation is less likely to occur if the metering edge contour is of a wedge configuration having a slope of no more than about 1 in 12 with respect to the outside diameter of the input-shaft. The low slope of the metering edge contour in the parking region makes it difficult to achieve the abovementioned highly accurate angular spacing of the metering edge contours, the latter spacing which controls valve operating angle and hence, not only valve noise, but also the steering gear parking efforts.
Several manufacturers seek to achieve the above described accuracy by grinding metering edge contours in special purpose chamfer grinding machines in which the input-shaft is supported on centres previously used for cylindrically finish grinding its outside diameter. Such machines have a large diameter grinding wheel, of a width equal to the axial extent of the metering edge contours, which is successively traversed across the edge of each input-shaft groove thereby producing a series of flat chamfers. In some cases each metering edge contour is constructed from a number of flat chamfers, usually one, two or even three flat chamfers per metering edge contour requiring, for example, as many as 36 separate traverses of the grinding wheel to manufacture the metering edge contours of a six slot input-shaft. Such a manufacturing method is therefore time consuming and expensive.
Other manufacturers adapt, for this purpose, grinding machines termed cam grinders, similar to those used for example in the manufacture of camshafts for automobile engines, thread cutting taps, and router cutters, wherein the workpiece is supported on centres and rotated continuously while being cyclically moved towards and away from a grinding wheel under the action of a master cam. The required amount of stock is progressively removed by infeeding of the grinding wheel during many revolutions of the workpiece. As in the case of chamfer grinding machines, a large diameter grinding wheel is used, which makes it impossible to grind that part of the metering edge contour towards the centreline of the groove where increasing depth would cause the grinding wheel to interfere with the opposite edge of the same groove. This steeply sloping and relatively deep portion of the input-shaft metering edge contour will henceforth be referred to as the "inner" metering edge contour and its geometry generally affects the on-centre region of the valve pressure characteristic. This portion is generally manufactured by means other than the chamfer or cam grinding machines just described which, for reasons stated, are only capable of grinding the "outer" metering edge contour. This previously described gently sloping wedge shaped portion of the metering edge contour determines the valve pressure characteristic at medium and high operating pressures, as well as determining the valve noise characteristic.
In the case of both chamfer and cam grinding methods described, the outside diameter of the hardened input-shaft is usually cylindrically ground on centres in an operation immediately prior to grinding the outer metering edge contours on these same centres. This is required because these centres are necessarily turned in the ends of the input-shaft workpiece prior to hardening and hence are no longer concentric with respect to its outside diameter after hardening, due to metallurgical distortion. However, for the same reasons, this method of processing inevitably results in the array of input-shaft grooves, machined on the same centres by milling or hobbing methods prior to hardening, being eccentric with respect to the input-shaft outside diameter.
Present manufacturers who chamfer grind metering edge contours by the methodology just described frequently true the sides of the axially extending input-shaft grooves using a small diameter, high speed grinding wheel, which is plunged radially into each groove. Such a truing operation, however, is not feasible in the case of cam grinding machines. Another method sometimes used to true the resulting eccentricity of the grooves after hardening is to re-true the centres in the input-shaft workpiece immediately after hardening by colleting the input-shaft in a fixture which locates on the outside diameter of the input-shaft adjacent to the grooves. Such re-trued centres can then be reliably used for subsequent cylindrical grinding of the outside diameter of the input-shaft as well as for grinding the metering edge contours. Whichever method is used for correcting the eccentricity of the array of input-shaft grooves, however, results in significant increases in time and therefore cost in the processing.
However the major disadvantage of processing both the input-shaft outside diameter and metering edge contours on centres is that the former of these two steps, that is cylindrically grinding the outside diameter of the input-shaft on centres, is much less efficient than the more commonly used centreless cylindrical grinding process. Centreless cylindrical grinding is generally more highly accurate than cylindrical grinding on centres, and can be readily implemented as a "through feed" or continuous process, leading to much reduced overall cycle times. Moreover, the expected accuracy gains of processing both the input-shaft outside diameter and the metering edge contours on centres may not always eventuate, and the array of metering edge contours may still be eccentric with respect to the input-shaft outside diameter. This residual eccentricity can be caused by damage to the fragile female centres of the input-shaft workpiece which are typically non-hardened.
It is apparent that many of the disadvantages of processing the hardened input-shaft on centres could be overcome by carrying out all such post-hardening operations in a centreless manner: that is centreless cylindrically grinding of the input-shaft outside diameter followed by centreless grinding of the metering edge contours. In the latter process the so-called control wheel would be moved in and out during grinding in a manner co-ordinated with the rotation of the input-shaft, so progressively grinding all contours around the outside periphery of the input-shaft. However, as referred to earlier, it is a necessary requirement that the valve operating angle be closely controlled, and the angular disposition of the points of intersection of the metering edge contour and the input-shaft outside diameter also accurately maintained. By using such a centreless grinding method for the input-shaft metering edge contours, the depth of any contour being ground would be determined by the distance between such contour and the diametrically opposite portion of the input-shaft outside diameter (corresponding to the point of contact with the control wheel). The depths of the metering edge contours so ground would vary not only in accordance with any errors in the contour grinding operation but also, in addition, absolute diametral errors resulting from the prior centreless grinding cylindrical operation carried out on the input-shaft outside diameter.
As far as is known such centreless grinding of metering edge contours has never been carried out commercially, perhaps due to this limitation associated with compounding of the errors.